1. Field of the Invention
The present invention relates to an image forming apparatus and a light intensity control method.
2. Description of the Related Art
A typical image forming apparatus forms a latent image on a drum-shaped photosensitive element by using a laser diode to irradiate the statically charged photosensitive element and then develops the latent image with developing materials, thereby forming an image. A typical laser diode emits from one to four or up to about eight laser beams from one semiconductor element.
In recent years, vertical cavity surface emitting lasers (VCSELs) have been used in practical applications, and it has been proposed that they are used in image forming apparatuses. A single chip of VCSEL can emit about 40 laser beams. Therefore, if such a VCSEL is used in an image forming apparatus to form a latent image, high-resolution, high-speed image formation, etc. will be achieved.
The high-speed formation of a high-resolution latent image cannot be achieved by simply using a VCSEL as a laser diode to form a latent image. For example, when a laser diode is used to form a latent image, a mechanism is needed that adjusts the light intensity of laser beams emitted by the laser diode to a target value. The VCSEL also needs a mechanism for adjusting the light intensity of the many laser beams emitted from its light emitting regions.
Because the number of laser beams emitted from a VCSEL is large, if light intensity control is made in the same manner for a VCSEL as it is for a laser diode emitting fewer number of laser beams, the time taken to control the light intensity is increased, which prevents any increase in the image forming speed. Furthermore, if part of the light intensity control over the laser beams is skipped, it is difficult to achieve high resolution.
Japanese Patent Application Laid-open No. 2009-1006 discloses a technology in which a current correction value is calculated depending on the output characteristics of the laser beam for each channel, and a common driving current that is used for every channel is corrected using the calculated correction value to obtain the light intensity of each laser beam. Thus, the light intensities of the laser beams are adjusted in an efficient manner and with a minimum increase in the circuit size.
When an image is formed using laser beams, problems occur related to shading. The laser beams strike a scanned surface after passing through an optical system that includes lenses, mirrors, etc. Therefore, the intensity of the laser beam varies over the scanned surface depending on image height (shading characteristics), and the shading characteristics affect the density of the formed image. Therefore, shading compensation is needed when an image is formed using laser beams.
For example, Japanese Patent No. 3466599 discloses a light intensity control technology in which the light intensity is assumed to be in direct proportion to the light emitting current. If light intensity correction data indicates a 10% increase in the light intensity at a certain image height, then the light intensity is increased by 10% by increasing the light emitting current by 10%. Japanese Patent Application Laid-open No. 2003-320703 discloses a shading compensation technology that is used with a light source unit that emits a plurality of beams of light.
However, it is difficult to implement accurate shading compensation using the technology disclosed in Japanese Patent Application Laid-open No. 2009-1006 because the threshold current is not adjusted even though adjustment of the threshold current is needed for accurate shading compensation.
In the technology disclosed in Japanese Patent No. 3466599, the light intensity is assumed to be in direct proportion to the light emitting current; however, the light intensity is not always in direct proportion to the light emitting current. Therefore, if the technology disclosed in Japanese Patent No. 3466599 is applied to the technology disclosed in Japanese Patent Application Laid-open No. 2003-320703, the light intensity of each laser beam may be not corrected to the target value. If the light intensity of each laser beam is set to an incorrect value, large differences may occur between the light intensities of the individual laser beams and, as a result, an image with periodical density unevenness (banding) may occur. Therefore, if the technology disclosed in Japanese Patent No. 3466599 is applied to the technology disclosed in Japanese Patent Application Laid-open No. 2003-320703, it is difficult to perform accurate shading compensation.